This invention is directed to the field of archery, and more specifically to compound bows of the type employing cams and control cables to achieve a programmed draw weight, and the latter being variable with draw length. The invention is more particularly concerned with improvements to such compound bows which make the bows more compact and streamlined, and which permit the archer to select the bow""s draw characteristic, and which increases the bow""s shooting performance.
A bow of this general type is described in my earlier U.S. Pat. Nos. 5,388,564 and 6,067,974. Those patents are incorporated herein by reference. Archery bows with programming means incorporated into them to regulate draw weight are also described in U.S. Pat. Nos. 3,854,417; 3,923,035; 3,486,495; and 4,287,867. These bows have means to regulate their draw weight so that a maximum pull weight is attained at an intermediate draw position, and with the draw weight dropping to some fraction of full draw weight at the full draw position. It is also an objective of such bows to transfer as much of the energy stored in the bow to the arrow, so that the arrow will fly faster and farther for a given draw weight. These goals have been difficult to achieve.
A number of compound bows have included one or more bowstring cams supported at the outer end of the spring limb or power limb. Typically one eccentric cam is provided on one limb, and there is a circular wheel at the end of the other limb. In addition, the bowstring cams currently used all are configured so that the radius increases on draw, i.e., the distance from the axis of the cam to the point of contact with the bowstring, i.e., the tangent with the bowstring. Accordingly, the radius decreases when the cam rewinds the bowstring. With this system, the rate at which the bow string moves forward becomes smaller as the bowstring approaches the fully released, i.e., brace position. This means that the bowstring does not accelerate the arrow optimally, and at least some of the energy stored in the power limb is wasted. Bows that employ bowstring cams are discussed, e.g., in Andrews et al. U.S. Pat. No. 6,082,346 and Despart et al. U.S. Pat. No. 6,474,324. In an ideal compound bow, all the energy stored in the power limbs should be transferred to the arrow to maximize the flight of the arrow. Also, any energy that remains in the bow will cause bow noise. Previous proposals for compound bows involving bowstring cams have not configured the mechanical advantage of the cam relative to the bowstring to maximize the energy transfer to the arrow.
Accordingly, it is an object of this invention to provide an improved compound bow that avoids the drawbacks present in the bows of the prior art.
It is another object to provide a bow that has increased performance for a given draw weight, and is quieter than current bows.
It is a further object to provide a compound bow that can be programmed easily to change its draw characteristics.
It is still another object of this invention to improve behavior of the bowstring cams of the compound bow.
One aspect of this invention involves an improvement to compound bows of the type that include a riser having an upper end and a lower end, with upper and lower resilient power limbs, i.e., spring limbs, that have their inboard ends affixed to the upper and lower ends of the riser, and synchronizing means, e.g., a cam and cable arrangement, for ensuring equal flexing of the upper and lower power limbs upon draw and release of the bowstring. In the bow according to preferred embodiments of the invention, there are upper and lower bowstring cams each being rotatably held at its axis on an outboard end of the power limb, and there are associated upper and lower power cam arrangements, each having one or more inboard cams rotatably held at a rigid portion of the riser, one or more outboard power cams affixed onto the associated bowstring cam to rotate with it, and one or more flexible inextensible cam cables extending over the periphery of the associated inboard and outboard power cams. The power cam arrangement is configured so as to cooperated and thus to determine draw characteristics of the bow. In embodiments of this invention, the bowstring cams are provided with a cam profile with a lobe or apex oriented such that the radius, from the axis thereof to the tangent with the bowstring, diminishes as the bowstring is drawn and increases as the bowstring returns after release from a drawn position to the full brace position. This changes the mechanical advantage of the cams on the bowstring after the bowstring is release so as to accelerate the bowstring and arrow. In other words, the rotational energy of the bowstring cam is transferred more efficiently to the arrow.
In a preferred arrangement, the bowstring cams may be forged of a lightweight metal. The bowstring cams may rotate over an angle exceeding 90 degrees between fully drawn and fully returned positions. The offset ratio of the bowstring cam, i.e., the size of the base circle relative to the lobe of the cam, may be on the order of ⅔, and in one preferred embodiment about 0.633. The profile of the bowstring cam achieves an optimal acceleration of the bow string and arrow, so that more of the bow""s energy is transferred as kinetic energy to the arrow. This also gives the bow a quieter action.
The above and many other objects, features, and advantages of this invention will present themselves to persons skilled in this art from the ensuing description of preferred embodiments of this invention, as described with reference to the accompanying Drawing.